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Full text of "Text Book Of Mechanical Engineering"

Appendix II.

781

jects over a table, as in Fig. 760, till they break by the weight of
the over-hanging portions, the results being seen in the diagram.
Unless the sand be properly prepared by riddling, treading, and
wetting, no amount of venting can make a good casting.

P. 12. Four-part Box.—However intricate in form a
casting may have to be, the difficulties of moulding may always
be met by the introduction of sufficient boxes or of loose pieces.
Fig. 761 shews a method of moulding a three-legged pot or
*-skillet/ practised at the Carron ironworks for an almost un-
known period.. Four boxes are used, numbered i, 2, 3, and 4,
which give also the order of removal, there being partings at
a a, bby and cc. The handles are pegged loosely to the pattern
from the inside, and are afterwards removed in a downward
direction, the core being struck on a rough iron body.

As evidence of what may be done when given enough
moulding-boxes and loose pieces, there was exhibited at Paris in
1889, by the Socie'te' Cockerill, a single casting of 10 tons
weight, representing three marine cylinders, with standards, bed-
plate, feed- and air-pumps.

P. 14. Plaster Patterns.—The use of these is not difficult
to explain, their introduction here being due to their great
similarity to loam patterns. Fig. 762 will indicate the process,
it being desired to make a plaster pattern for a dome cover.
Firstly, a supporting mound A is made by the rotation of board
B over moist plaster-of-Paris, the vertical spindle hanging from
a wall bracket When dry, the mound is painted with shellac
varnish, and the thickness piece described by the board c. The
pattern D, thus formed, is afterwards removed, varnished, and
used exactly as a wooden pattern would be; and with care may
serve for a large number of impressions.

A further use of plaster occurs in moulding thin flat objects.
Imagine a flat cover, Fig. 762^, say for a sand-box. Firstly, a
wooden pattern, being impressed in sand as at A, and the parting
made, the upper box is filled with plaster. When the plaster sets,
the sand is removed, the plaster surface varnished; and the
bottom box similarly treated, as at B. Secondly, the boxes are
separated and the pattern removed. Thirdly, a new pair of boxes
is provided of the same dimensions, and, taking each box sepa-